AwesomePlayer: fix concurrent pause and teardown

There was a race condition if a pause request was received
while we were executing a teardown event. Although we hold
a mutex while processing the teardown event, there is a step
of the prepare sequence where we release the mutex if streaming.
In this case, the pause request is executed but ignored because
the player state is still preparing. At the end of the teardown
event processing we restore previous playback state and resume.

The fix consists in clearing the saved teardown playback state
when a pause request is received while processing a teardown event.

Bug: 11225491.
Change-Id: If0e61855ce5a336322f1ba8e5559bdc190beeb76